In a typical Fluid Catalytic Cracking (FCC) unit, the absorber off-gas, also known as dry gas, contributes to approximately one-third of the refinery fuel gas production. Dry gas is the common name for the absorber off-gas stream that contains all the gases with boiling points lower than ethane. The remaining stream is known as the FCC dry gas. A typical dry gas stream contains 5 to 50 wt % ethylene, 10 to 20 wt % ethane, 5 to 20 wt % hydrogen, 5 to 20 wt % nitrogen, about 0.05 to about 5.0 wt % of carbon monoxide and about 0.1 to about 5.0 wt % of carbon dioxide and less than 0.01 wt % hydrogen sulfide and ammonia with the balance being methane and other impurities.
The ethylene and heavier hydrocarbons in the dry gas are valuable components. Ethane can be good feed source for an ethane cracking facility for ethylene production and ethylene can be recovered for polyethylene production. Currently most ethylene and ethane in the dry gas is burned instead of recovered because the off-gas contains so many contaminants that are uneconomical to remove. However, dry gas streams still contain attractive quantities of ethylene, ethane and heavier components if recovery could be made economical.
An FCC unit that processes 7,949 kiloliters (50,000 barrels) per day will generate and burn as much as 181,000 kg (200 tons) of dry gas containing about 36,000 kg (40 tons) of ethylene and 14,400 kg (16 tons) of ethane as fuel per day. Because a large price differential exists between fuel gas and pure ethylene or steam cracker feed it would be economically advantageous to recover this ethylene and ethane from dry gas. However, the dry gas stream contains dilute impurities that can poison catalyst or impair ethylene recovery, but their removal is not economically justified by gas recovery systems. Impurities in dry gas include: hydrogen sulfide, mercaptans, carbonyl sulfide, carbon dioxide, carbon monoxide, nitrogen, nitrous oxides, oxygen, acetylene, ammonia, chlorides, and arsenic.
Accordingly, it is desirable to provide apparatuses and processes for the removal of impurities from dry gas to allow recovery and use of ethylene and ethane in a safe and a cost-effective manner.